Uranium surface preparation for electroless nickel plating

ABSTRACT

THE QUALITY OF NICKEL PLATING APPLIED TO URANIUM BODIES BY THE ELECTROLESS PLATING PROCESS IS SUBSTANTIALLY IMPROVED IF, AFTER DEGREASING, PICKLING AND ETCHING, THE URANIUM BODIES ARE TREATED WITH A SATURATED SOLUTION OF PALLADIUM CHLORIDE OR A HOT SOLUTION OF TETRASODIUM PYROPHOSPHATE AND RINSED BEFORE IMMERSION IN THE ELECTROLESS NICKEL PLATING SOLUTION.

United States Patent Office 3,700,482 Patented Oct. 24, 1972 3,700,482URANIUM SURFACE PREPARATION FOR ELECTROLESS NICKEL PLATING Germaine F.Jacky, Portland, Greg, and Richard D.

Ehrlich, Oakland, Calif., assignors to the United States of America asrepresented by the United States Atomic Energy Commission No Drawing.Filed Dec. 1, 1970, Ser. No. 94,162

Int. Cl. C23c 3/00 U.S. Cl. 117-50 3 Claims ABSTRACT OF THE DISCLOSUREThe quality of nickel plating applied to uranium bodies by theelectroless plating process is substantially improved if, afterdegreasing, pickling and etching, the uranium bodies are treated with asaturated solution of palladium chloride or a hot solution oftetrasodium pyrophosphate and rinsed before immersion in the electrolessnickel plating solution.

CONTRACTUAL ORIGIN OF THE INVENTION The invention described herein wasmade in the course of, or under, a contract with the United StatesAtomic Energy Commission.

BACKGROUND OF THE INVENTION This invention relates to a process forplating nickel on uranium metal. More specifically, this inventionrelates to an improvement in a process for plating nickel on the surfaceof a uranium body by the electroless nickel plating process.

The nature of uranium metal is such that before it can be used innuclear reactors it must be either alloyed or protected by cladding withanother metal such as aluminum. It is important that these fuel elementsexhibit certain characteristics, such as good heat transfer, dimensionaland mechanical stability, resistance to corrosion and mechanicalproperties to withstand operating stresses. The cladding of uranium withaluminum results in the formation of solid state diffusion bonds, whichare uranium and aluminum compounds having varying compositions, at theinterface between the two metals. Generally, the uranium and aluminumcompounds so formed do not have these desired properties. However, byplating a thin layer of nickel on the uranium prior to cladding theuranium with aluminum, the formation of these undesirableuranium-aluminum compounds is prevented and a diffusion bond is formedhaving the desirable characteristics.

The usual method of coating the nickel on the uranium is by means ofelectroplating. However, in electroplating the current density on thesurface of the piece being plated is much greater in corners than onfiat areas, leading to a plating which is thick on the corners andthinner in the flat areas. In addition, electroplating requires bulkyelectrical equipment, such as transformers, rectifiers and terminalboards, and also special plating racks which are necessary to maintaingood electrical contact as well as to provide physical support for thearticle being plated.

Another method by which nickel can be plated on the uranium is bychemical deposition. In chemical deposition the nickel deposits are veryuniform in thickness, regardless of the geometry of the article beingplated. In addition, the equipment necessary for such plating needconsist only of racks to provide physical support for the article to beplated and tanks for holding the plating solution. Chemical depositionof nickel is performed in hot aqueous solutions which contain nickelions, a reducing agent such as sodium hypophosphite, and variousadditives to optimize the chemical reactions which take place in thebath. Either acidic or alkaline baths may be used. However, chemicaldeposition of nickel on uranium often results in a plating having pooradhesion. After immersion of the uranium in the plating bath, anexcessively long period of time is occasionally required before thestart of plating deposition. This time interval, hereinafter calledinitiation time, often ranges from 10 minutes to 30 minutes or more. Along initiation time causes the uranium to turn brown, which indicatesan oxidation of the surface. This oxidation prevents good adhesion ofthe nickel plating to the uranium metal and results in a plate which iseasily removed when probed with a sharp tool.

Before nickel plating by chemical deposition can be performed on theuranium, the metal must be pretreated so that the initiation time willbe short and the plating will adhere properly. The usual pretreatmentconsists of degreasing the uranium in trichloroethylene or in a hotcaustic solution and pickling the metal in concentrated nitric aciduntil the surface is deoxidized. The metal is then electrolyticallyetched and pickled in a solution of nitric acid and rinsed in waterseveral times prior to immersion in the electroless nickel plating bath.

SUMMARY OF THE INVENTION We have found that, by proper pretreatment ofthe uranium bodies immediately prior to immersion in the plating bath,we are able to reduce substantially the initiation time of the platingof nickel on the uranium surface and are thus able to substantiallyimprove the adhesion of the nickel plate to the uranium.

In the process of this invention, the uranium body is degreased,pickled, etched and pickled and rinsed several more times before it isdipped in a solution of palladium chloride, dipped in a solution ofsodium hypophosphite, rinsed and dipped again in the sodiumhypophosphite before it is immersed in the electroless nickel platingbath. The uranium body may also, after the pickling and rinsing steps,be dipped into a hot solution of tetrasodium pyrophosphate beforeimmersion in the nickel plating bath. By the method of thispretreatment, the initiation time of plating of nickel on uranium can bereduced to as little as about 15 seconds and the adhesion of the nickelplate to the uranium metal improved substantially over that attained bythe prior art methods.

It is therefore one object of this invention to provide an improvedmethod for the plating of nickel on uranium.

It is another object of this invention to provide a method fordecreasing the initiation time of plating nickel on uranium.

Finally, it is an object of this invention to provide a method ofplating nickel on uranium which results in better adhesion of thenickel.

DESCRIPTION OF THE PREFERRED EMBODIMENTS These and other objects of thisinvention may be attained by degreasing the uranium body to be nickelplated in trichloroethylene or in a caustic such as 5% NaOH at to C.,pickling the body in 8 to 12 M HNO, at 70 to 90 C. for 1 to 5 minutesuntil deoxidized, rinsing in water, etching the body by passing 50 to 70amps/sq. ft. of current through it for 5 to 6 minutes while in asolution of 5.2 to 6.0 M H PO and 0.25 to 0.3 M HCl, rinsing in water,pickling in 8 to 9 M HNO';, at 40 to 45 C. for 2 to 3 minutes, rinsingin water, pickling in 8 to 9 M HNO at 15 to 25 C. for 2 to 3 minutes,rinsing in water and then further treating the uranium body by dippingit for 30 seconds in a 0.4 M tetrasodium pyrophosphate solution at 70 C.or by dipping the body for 15 seconds in a saturated solution ofpalladium chloride,

followed by a 30-second dip in 0.225 M sodium hypophosphite solution, arinse and another clip in the hypophosphite solution immediately priorto immersion of the uranium body in the electroless nickel plating bath.

It is important that the bodies to be plated are thoroughly rinsed toprevent any carry-over of palladium chloride to the nickel platingsolution, since the smallest amount of palladium chloride or palladiumwill cause a rapid disintegration of the plating solution.

A plating bath with which excellent results have been attained has thefollowing composition:

Ni(C2H302)2'4H2O M NaH PO -H O M 0.600 N21 C H O '5 /2H O M HC H O M0.65 Add NI-I OH to give pH 9.0.

Sodium lauryl sulfate gm./l. 0.1

The initiation of plating of uranium bodies treated with palladiumchloride in the above nickel plating bath at 70 C. was within 15 secondsand the uranium does not change color before plating commences.

When plating uranium bodies which have been treated with pyrophosphate,initiation is increased to about 2 minutes and the bodies turn to ayellow-brown color before plating commences; however, adhesion of thenickel plate to the uranium substrate was found to be as good as thatattained by the use of the palladium chloride dip.

Samples of uranium metal plated with nickel which have been prepared bythe above method have been tested by boiling in water for 16 hours afterbeing plated. No spalling can be induced on the samples by probing witha sharp tool before or after fracturing the samples. When sufiicientforce is exerted, the nickel can be pried off, but it leaves a cleanuranium surface, indicating that the uranium substrate is broken,indicating a good metal-tometal bond has been established.

The following examples are given as illustrative of the process of thisinvention and are not to be taken as limiting the scope or extent of theinvention.

Example I Four washer-shaped uranium bodies labeled BP, BQ, BR and BTwere nickel plated by the following method: degreased in a 5% solutionof NaOH at 80 C. for 2 minutes; pickled in M HNO for 3 minutes; rinsedin tap water; etched in a solution of 5.5 M. H PO and 0.25 M HCl bypassing 12 amps through the bodies for 5 minutes and 25 amps for 20minutes; rinsed in tap water; pickled in 8 M HNO at 40 C. for 1 minute;rinsed in tap water; pickled in 8 M HNO at 20 C. for 1 minute; alkalinerinse (a tap water rinse is now considered to be sufiicient); dipped ina saturated solution of PdCl for l 5 seconds; dipped in 0.225 M sodiumhypophosphite for 30 seconds; and rinsed in tap water and again dippedin 0.225 M NaH PO for 30 seconds.

The treated bodies were then immersed in a nickel plating solution at atemperature of 70 C. which had the following composition:

Ni(C H O -4H O M 0.121 NaH PO -H O M 0.600 Na C I-I O -5 /2H O M HC H OM 0.65 Add NH OH to give pH 9.0.

Sodium lauryl sulfate gm./l. 0.1

Initiation of plating commenced at about 15 seconds and the bodiesremained in the plating solution for 2 hours and 25 minutes. Whenremoved, the surfaces of the samples were very smooth with little or nolustre.

Sample BP was boiled in water for 16 hours with no sign of platingfailure.

Samples BP and BR were also heated in a muffle furnace at 600 C. for 30minutes in an air atmosphere with no sign of plating failure at the endof that period.

Example II Three unmarked samples were treated as described in ExampleI, except that after the alkaline rinse the samples were dipped in 0.40M tetrasodium pyrophosphate rinse for 30 seconds, dried for 30 secondsand then dipped again into the 0.4 M tetrasodium pyrophosphate solutionat 70 C. for 30 seconds before being immersed in the nickel plating bathpreviously described at 70 C.

The samples turned brown almost immediately upon entering the bath andplating did not start to appear upon the samples for 2 minutes. A fullcovering was not attained on the samples for 4 minutes after entry intothe bath. The samples remained in the plating bath for 2 hours and 15minutes, after which they were removed for inspection. The samples weredull gray in color with streaks that had some lustre and smoothness.

The plating was tested by boiling the samples in water for 16 /2 hours.The only noticeable effect was discoloration of the plate, which variedbetween the samples. There was no sign of water penetration. One samplewhich was broken showed no sign of spalling and quite excellent plateadhesion to the uranium substrate. The samples were then tested byplacing them into a mufile furnace and heating them in air to 600 C. for30 minutes. At the end of this period, one sample had several smallblisters about the inside hole which were throught to be due to stresspoints. The other sample had a large number of small bubbles on thesides which may have been caused by a poor sample.

It can be seen from the above examples that quite good nickel platingscan be attained on uranium substrates by the method of this invention.

It will be understood that the invention is not to be limited to thedetails given herein but that it may be modified within the scope of theappended claims.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. A method of pretreating uranium bodies for improved electrolessnickel plating comprising:

degreasing said bodies;

pickling said bodies in 8-l2 M nitric acid;

rinsing said bodies in water;

electrolytically etching said bodies in solution containing 5.2 to 6.04M H PO and 0.25 to 0.3 M HCl; rinsing said bodies in Water;

pickling said bodies in hot 8-9 M nitric acid;

rinsing said bodies in Water;

pickling said bodies in cold 8-9 M nitric acid;

rinsing said bodies in water; and contacting said bodies with a solutionselected from the group consisting of palladium chloride and tetrasodiumpyrophosphate, wherein said bodies are pretreated for plating andfurther treating said bodies contacted with palladium chloride bycontacting said bodies with sodium hypophosphite, rinsing said bodies inwater, and contacting said bodies with sodium hypophosphite to removeany palladium chloride continued thereon.

2. The method of claim 1 wherein said bodies are degreased intrichloroethylene, pickled in 8 to 12 M nitric acid at 70 to 90 C.,etched by passing 50 to 70 amp/sq. ft. of current through said bodiesfor 5 to 6 minutes, said hot nitric acid solution is at 40 to C., saidcold nitric acid solution is at 15 to 25 C., said palladium chloridesolution is a saturated solution and said bodies remain therein for 15seconds, said solution of sodium hypophosphite is an aqueous solutioncontaining 0.225 M sodium hypophosphite and said bodies remain thereinfor 30 seconds.

3. The method of claim 1 wherein said bodies are degreased intrichloroethylene, pickled in 8-12 M nitric acid at 70 to 90 C., etchedby passing to amp/sq. ft. of current through said bodies for 5 to 6minutes, said hot nitric acid solution is at 40 to 45 C., said coldnitric References Cited UNITED STATES PATENTS 9/1967 Anderson et a1.117-50 9/1966 Lundquest et a1. 2041.5

6 3,574,664 4/1971 Feldstein 117--47 R 2,854,738 10/1958 Gray 204-15 X3,573,120 3/1971 Waldrop et a1. 204-32 RX 5 RALPH S. KENDALL, PrimaryExaminer C. WESTON, Assistant Examiner U.S. c1. X.R. 117-47 R, 130 E;176-82; 204-15, 32 R

